Malignant tachyarrhythmia, for example, ventricular fibrillation, is an uncoordinated contraction of the cardiac muscle of the ventricles in the heart, and is the most commonly identified arrhythmia in cardiac arrest patients. If this arrhythmia continues for more than a few seconds, it may result in cardiogenic shock and cessation of effective blood circulation. As a consequence, sudden cardiac death (SCD) may result in a matter of minutes.
In patients at high risk of ventricular fibrillation, the use of an implantable cardioverter-defibrillator (ICD) system has been shown to be beneficial at preventing SCD. Conventional ICD systems include an ICD that is coupled to one or more electrical stimulation leads placed on or within the heart. If a malignant tachyarrhythmia is sensed, the ICD delivers a cardioversion/defibrillation shock via a defibrillation electrode of the electrical lead to restore the heart to its normal rhythm. Owing to the inherent surgical risks in attaching and replacing electrical leads directly within or on the heart, subcutaneous ICD systems have been devised to provide cardioversion or defibrillation shocks to the heart using an electrical stimulation lead implanted subcutaneously outside the torso of the patient (e.g., outside of the ribcage).
Subcutaneous ICD systems may require an output of around 75-80 Joules (J) of energy to provide effective defibrillation therapy. As a result, subcutaneous ICDs may require larger batteries and more storage capacitors than transvenous ICDs. As such, the subcutaneous ICDs are generally larger in size than transvenous ICDs. The large size of the subcutaneous ICD may compromise patient comfort, decrease system longevity and/or increase cost of the system. In addition, subcutaneous ICD systems are incapable of delivering anti-tachycardia pacing (ATP), which is a standard therapy in transvenous ICDs to terminate ventricular tachyarrhythmias without providing a cardioversion or defibrillation shock.